DNA single- and double-strand breaks arise in the genome due to oxidative DNA damage and treatment with DNA damaging agents, including ionizing radiation. Understanding the fundamental mechanisms of the repair of these lesions will increase our understanding of cancer risk and treatment. We have recently shown that DNA polymerase beta (Pol beta) is critical for double-strand break (DSB) repair, in addition to its role in base excision repair. The broad, long-term objective of the proposed research is to provide mechanistic insight into the function of Pol beta in DNA repair in a whole organism. The focus of the first aim is on the specialized cellular process of meiosis. We will test two hypotheses. The first is that the interaction of Pol ss with other proteins is important for meiosis and strand break repair. We will use a combined genetics and biochemical approach to test our hypotheses. Mouse models which are either Pol beta protein partner interaction-defective and which do not express the polymerase activity of Pol beta will be constructed and analyzed in vivo for their ability to function in DSB repair. Biochemical assays of Pol beta, its protein partners, and appropriate DSB repair DNA substrates will be analyzed in vitro in order to understand the specific function of Pol beta in DSB repair. The focus of the second aim is on the role of Pol beta in the processing of DNA repair intermediates. We will test the hypothesis that Pol beta is critical for processing repair intermediates during both base excision repair (BER) and break repair. Our preliminary data suggest a role for the polymerase activity of Pol ss in the prevention of toxic accumulation of BER intermediates and also in Non-Homologous End-Joining (NHEJ). We will characterize these processes in detail. Because there are several germline polymorphisms within the Pol B gene and the genes that encode proteins that interact with Pol beta in DSB repair, the proposed project is important for human health in that it has the potential to further our understanding of how these polymorphisms contribute to cancer risk, genomic instability, infertility and cancer treatment. In addition, fundamental mechanistic insight into the process of break repair will be obtained.